1. Field of the Invention
The present invention relates to a semiconductor integrated circuit for controlling a transition of a plurality of switch transistors, which are used for supplying power to a plurality of circuit cells and cutting off the supply of power to the circuit cells, from a turned-off state to a turned-on state and relates to a power-supply control method for controlling such a transition.
2. Description of the Related Art
A MTCMOS (Multi-threshold Complementary Metal Oxide Semiconductor) technology is known as a technology for controlling switch transistors, which are used for supplying power to a circuit and cutting of the supply of power to the circuit.
The threshold voltage of a transistor employed in a logic circuit or the like is a typical design value. In general, it is necessary to reduce the threshold voltage so that no signal delay is generated due to a decreased power-supply voltage and/or a miniaturized device. If the threshold voltage of a transistor employed in a logic circuit or the like is small, a leak current flowing through the transistor is large. In accordance with the MTCMOS technology, a transistor as a power-supply switch is designed to have a large threshold voltage for a circuit in a stopped state in comparison with a transistor employed in a logic circuit or the like and is used for breaking a leak current path of the logic circuit or the like in order to prevent its power from being consumed wastefully.
In an application of the MTCMOS technology to a circuit block, local voltage lines referred to as the so-called virtual VDD line and the so-called virtual GND line are provided locally in the circuit block. The local voltage lines are each connected to a real voltage line and a real reference-voltage line respectively through a switch transistor for supplying power to the circuit block and cutting off the supply of power to the circuit block. Referred to as a real VDD line and a real VSS line respectively, the real voltage line and the real reference-voltage line are common global voltage lines outside the circuit block.
A switch transistor is provided between the real VDD line and a functional circuit which is started and stopped repeatedly. As an alternative, a switch transistor is provided between the real VSS line and such a functional circuit. As another alternative, a switch transistor is provided between the real VDD line and such a functional circuit whereas a switch transistor is provided between the real VSS line and the functional circuit. Normally, the switch transistor provided between the real VDD line and a functional circuit is a p-channel metal oxide semiconductor (PMOS) transistor whereas a switch transistor provided between the real VSS line and a functional circuit is an n-channel metal oxide semiconductor (NMOS) transistor.
Operations to activate and stop a functional circuit included in an MTCMOS applied block are controlled by a circuit included in a non-MTCMOS applied block which receives power from the real VDD line and the real VSS line, entering an operating state at normal times, after a semiconductor integrated circuit has been activated. The non-MTCMOS applied block also includes the switch control circuit which controls operations to turn on and off the switch transistor for supplying power to circuit cells and cutting off the supply of power to the cells. In addition to the switch conduction control circuit, the non-MTCMOS applied block also include circuits, such as a clock generation circuit and another repeater buffer, used mainly for controlling the entire integrated circuit (IC) and storing data representing input/output signals.
If the stopped time of the functional circuit in the MTCMOS applied block is long, it is quite within the bounds of possibility that the local voltage line such as the virtual VSS line is electrically charged with a leak current flowing from another internal circuit and raised to a high electric potential close to the real VDD line. Thus, when a power-supply cutting-off switch transistor is turned on at the time the functional circuit in the MTCMOS applied block is reactivated, electrical discharging of the virtual VSS line causes an accidental current to flow to the real VSS line. This accidental current is referred to as, for example, a rush current. As the rush current flows to the real VDD line, the current becomes a positive noise potential and propagates to a non-MTCMOS applied block adjacent to the MTCMOS applied block.
A phenomenon similar to that described above may occurs on the real VDD line. Since the accidental current flows from the real VDD line, a negative noise potential appears, which drops the potential on the real VDD line abruptly.
In either case, these noise voltages caused by power-supply noises propagate to circuits operating in an adjacent circuit block and cause a steep decrease of the power-supply voltage amplitude, giving rise to a delay effect such as generation of an operation delay as a result. The circuits operating in an adjacent circuit block include a clock generator circuit and/or a repeater buffer.
As a countermeasure against the power-supply noises, Philippe Royannez etc., “90 nm Low Leakage SoC Design Technique for Wireless Application,” 2005 IEEE International Solid-State Circuits Conference, DIGEST OF TECHNICAL PAPERS, P138 (referred to as non-patent document 1), for example, discloses a technology according to which a plurality of PMOS switch transistors are connected in parallel between the global real VDD line and the local virtual VDD line and control signals each applied to the gate of one of the transistors are each delayed in order to gradually reduce the impedance of connection between the global real VDD line and the local virtual VDD line.